Because it hides imperfections better, semi gloss or other glossy paint really do not hide any little blurbs as well.

If your painting your walls afterward heres a nice tip for quickly rolling the ceiling. With the roller edge get right into the corner where the ceiling meets the wall, that what all you need to edge is the corners. If theres any drips just smooth those out with a brush.

When rolling a ceiling always maintain a WET EDGE, meaning dont let it dry and go as fast as possible. If you roll over a dry edge some marks and lines might be visible afterward which would result in having to do more coats.